1. Field of the Invention
This invention relates generally to the art of thin film device processing and fabrication. More specifically, the invention relates to the fabrication of Organic Light Emitting Diode based displays and other electronic devices which use selective deposition.
2. Related Art
Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of optoelectronic elements (“elements”) such as arrays of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has come into vogue for certain applications. Examples of other elements/devices using organic materials include organic solar cells, organic transistors, organic detectors, and organic lasers. There are also a number of bio-technology applications such as biochips for DNA recognition, combinatorial synthesis, etc. which utilize organic materials.
An OLED is typically comprised of two or more thin at least partially conducting organic layers (e.g., an electrically conducting hole transporting polymer layer and an emissive polymer layer where the emissive polymer layer emits light) which are sandwiched between an anode and a cathode. Under an applied forward potential, the anode injects holes into the conducting polymer layer, while the cathode injects electrons into the emissive polymer layer. The injected holes and electrons each migrate toward the oppositely charged electrode and recombine to form an exciton in the emissive polymer layer. The exciton relaxes to a lower energy state by emission of radiation and in process, emits light.
Each of the OLEDs can be a pixel element in a passive or active matrix OLED display. Such pixels can be arranged in a row-column fashion and would be addressed and switched on/off differently depending upon whether the display was active or passive matrix. In the passive matrix case, each pixel is not individually addressed by a switch, but rather pixels are addressed using a combination of row lines and a column lines. In active-matrix displays, each pixel is controlled by its own switch (e.g. transistor based) which allows it to remain on until switched off.
FIG. 1 illustrates a cross-section of a typical OLED pixel. OLED pixel 100 includes a patterned anode layer 102 (typically the columns) that are patterned into stripes on top of a substrate 101. Anode layer 102 is typically composed of a transparent conducting oxide such as ITO (Indium Tin Oxide) or Fluorine-doped Tin Oxide. After anode patterning (usually via a photolithography and etching step), metal lines (not shown) may be patterned upon part of the anode pattern using methods known to those of ordinary skill in the art (e.g. metal deposition, photolithography and/or etch).
FIG. 1 depicts a metal cathode layer 104 that is laid down, typically on top of various polymer layers 108 and 109, to provide electrical connection for the active pixel area formed consequently. The pixel 100 illuminates under an application of a forward biased voltage as discussed above. The polymer layers 108 and 109 are typically formed by depositing a gel or liquid substance on the device and then spinning the device to spread out the deposited substance. This is referred to as spin coating. In other cases, the gel or liquid can be deposited and simply evaporated/dried for a period of time. In selective deposition techniques, such as inkjet printing (described below) pockets are formed into which the gel or liquid can be deposited drop by drop.
In the configuration shown, polymer layer 109 is usually an emissive polymer layer. Typically, polymer layer 108 is a conducting polymer layer which is also called a hole transport layer because it transports holes from the anode layer into the emissive polymer layer 109. The order in which layer 108 and 109 are deposited would be reversed if layer 104 were the anode layer and layer 102 the cathode layer. As mentioned above, one method of fabricating polymer layers is to drop a liquid or gel onto the lower electrode layer or other underlying surface and allow it to dry. The polymer liquid is deposited in drops and allowed to spread out on the exposed surface of the anode layer and then dry into a film. In the case of selective deposition such as inkjet printing, however, pockets that are formed by photo-resist banks can often affect the spreading/flow/drying and affect the shape of the film.
While spin coating of polymers may be appropriate for monochrome light emission, it is generally not used when a pattern of pixels of different colors is desired. For instance, if a particular pixel is to emit red light, a red emissive polymer material would be deposited in the region of that pixel, while for a blue emission a blue emissive polymer material would be deposited. In such cases, it is typical to have layer of photo-resist that form pockets defining discrete deposition regions to confine the deposited liquids and avoid intermixing of different colors as shown in FIG. 2.
FIG. 2 illustrates an anode line 210 that has a layer of photo-resist 220 above it. The photo-resist 220 is patterned (in case of photolithography the photo-resist is developed away, while for screen printing, the photo-resist can be deposited where needed) to define a pocket 230 where the anode 210 is exposed. The pocket 230 defines an individual pixel region and defines an area upon which polymer layers can be deposited. Unlike the spin coating case, each pocket, thus formed, is isolated from the pocket of another pixel location in that the same substance dropped therein does not spread and overlap with that other pocket. FIG. 2 shows a cross-section cut in the same direction as the direction in which anode line 210 and other anode lines are patterned. A cross-section in the transverse direction to that shown in FIG. 2 would also show photo-resist with the same shape as photo-resist 220. This layer of photo-resist patterned as shown define individual pockets as illustrated in FIG. 3. The extra layer of photo-resist needed to define pockets typically has the shape shown in photo-resist 220 such that no portion of the photo-resist 220 overhangs into the pocket 230. The polymer(s) is/are dropped into the pocket 230 and allowed to dry or be evaporated.
In conjunction with these pockets, the process of inkjet printing has been used especially where a specific pattern of pixels are desired. For instance, in an OLED display where some individual pixel locations emit red light, while others emit green and yet others emit blue, inkjet printing would allow such a pattern to be created. FIG. 3 shows an inkjet printing system for an OLED that is to have RGB (red, green and blue) patterned pixels. The cross-section section shown in FIG. 3 is perpendicular to the cross-section shown in FIG. 2 and is taken in a direction perpendicular to the direction in which anode lines 360 are patterned. A print head 350 contains different polymers or other organic or organometallic substances including a red light emissive polymer substance, a green light emissive polymer substance and a blue light emissive polymer substance. These three polymers substances are delivered, respectively, via nozzles 332, 322 and 312. (although it is possible to deposit a different material from each nozzle, industrial printers are not designed like that. Usually all nozzles are used to deposit one material and the different materials are deposited sequentially). When delivered, the polymer substances drop onto pockets formed over an anode layer 360 (or onto the conducting polymer layer 330 which may also have been deposited using an inkjet head similar to the inkjet head 350 shown in FIG. 3). When the red, green and blue emissive polymer substances dry or evaporate, they form red polymer layer 334, green polymer layer 324 and blue polymer layer 314, respectively. One example shown is pocket 390 which has a green emitting polymer dropped therein to form green emitting polymer layer 324 (top view).
The red, green and blue pixels are patterned into a pattern using photo-resist layer 340 which will ensure that the correct polymer substances are deposited into only that specific pocket of the OLED display which it defines. The pattern shown is merely illustrative of one of many possible patterns of red, green and blue pixels. The use of a photo-resist layer to define pockets for inkjet printing is disclosed in published patent application No. U.S. 2002/0060518 A1 entitled “Organic Electroluminescent Device and Method of Manufacturing Thereof”. As shown, the photo-resist 340 has a normal slope such that the top of the photo-resist 340 does not overhang the pocket. The typical height of the photo-resist banks is between 3 and 10 microns (when measured from the base) as currently practiced in the industry. Photo-resist banks are of such height to ensure that deposited drops do not spill over the walls of the photo-resist. However high photo-resist banks can lead to non-uniform and concave drying patterns. The profile, when dried from a liquid drop, will not be very flat or uniform.
As can be observed, the drying pattern is very non-uniform and shows a piling up on the edges of the drop 400 in FIG. 4. This is due to the difference in the rate of evaporation in different regions of the drop 400. This difference causes the substance to move towards the edges of the drop 400 from the middle, and hence the ultimate deposition of more of the substance at the edge than in the middle. This phenomenon is usually referred to as the Marangoni effect. A common example of this phenomenon is the stain left behind by drying of a coffee drop which shows more prominence (is darker in color) on the edges of the stain than in the center.
When there are normal sloping banks as in the case of drop 410, there is still substantial non-uniformity in the profile of the drop when dried. There is pileup at the edges which affects the useful part of the device. This pileup is due in part to the “pinning” of the liquid against the wall of the photo-resist bank. The liquid volume often exceeds the height of the banks as shown. The higher the pinning line is, the more pile-up at the edges when compared to the rest of the dried film. With the thicker dried film, the thickness of the layer 330 and 334 thus increases (see FIG. 3), the current through that part of the film decreases leading to less light being emitted from the part of the pocket with thicker layer(s) 330 and 334. In addition the higher current density in the thinner part of the film would lead to faster degradation of the sample and hence a shorter lifetime of the device/product.
It would be desirable to optimize the height of the banks such that the drying polymers are more uniform in thickness and thus have a flatter profile than is traditionally observed.